Various communication lines which operate within each others' vicinity may induce signals on each other. For instance two communication lines such as two VDSL2 lines which are collocated next to each other induce a signal in each other. Such signal is called crosstalk and has an influence on the quality of communication on these lines. Due to the induced crosstalk and noise from other sources in the surroundings of the communication line, the data transported on these lines may be affected or corrupted by the crosstalk and noise. Corrupted data needs to be corrected or transmitted again before the receiver can process the data. By reducing the crosstalk induced on a communication line or compensating the crosstalk induced on a communication line, the amount of corrupted data may be reduced, the need for error corrections or retransmissions is reduced, and the rate at which information can be reliably communicated is increased.
Various prior art solutions exist which are able to reduce the effects of crosstalk on communication lines and in particular on digital subscriber lines. These solutions are typically based on performing measurements on communication lines and adapting the operational conditions of the lines accordingly. Such measurements provide information concerning the crosstalk between various communication lines which can be used as an indication for line quality, attainable data rate, reliability of the communication channel, etc. The crosstalk channel coefficients can be used to reduce the effects of crosstalk on the communication line or to compensate for the crosstalk in order to remove the problem almost entirely.
Each communication line is a possible disturber line which induces crosstalk in one or more victim lines. By transmitting test signals across all the lines, it is possible to determine the influence of each offender line on the victim lines. The test signals can be characterized by the way in which power is allocated to one or more tones or frequencies. For instance a test signal may be transmitted using a particular power level over a small frequency range. The victim line may notice this power in that frequency range and be able to determine the amplitude of that power. The amplitude of the induced influence of crosstalk on a particular line is a good reference to determine how strong particular crosstalkers are or which frequencies or tones are susceptible to the crosstalk of certain crosstalkers. In particular, if each possible communication line which may induce crosstalk (also called a crosstalker) emits a distinct power spectrum as test signal, it is possible to detect which crosstalker or crosstalkers are inducing noise on a given victim line.
The information obtained by such measurements can then be used to group communication lines in physical groups or logical groups or binders, define the transmit spectra on the communication lines, etc. This enables a network operator to configure lines in such a way that crosstalk is avoided as much as possible or that the effects of crosstalk are reduced. For instance by selecting a transmit spectrum on a victim line in such a way that frequencies or tones with strong crosstalk influences from offender lines are avoided, the quality of the victim line is improved and the crosstalk is reduced.
A problem with the above given approach is that it is difficult to adjust transmit power spectra of lines in operation. Such changes reduce the throughput of the line significantly and may even lead to a service outage for a user when a line has to be reconfigured. Thus the above described technique can only be used for upcoming lines which may also be called joining lines and may not be able to handle significant changes in crosstalk once the line is in operation. In addition, the measurements can be performed relatively easily by transmitting a particular power on the possible disturber lines. However by altering the transmit power or power spectral density (PSD), it is only possible to measure or estimate the amplitude of the crosstalk. The phase of the crosstalk signal cannot be assessed using such a technique. This means that the exact crosstalk channel, described mathematically by complex coefficients having both magnitude and phase, is not known.
Precoding techniques are based on transmitting an additional signal on top of the data signal which is used to compensate the crosstalk on a victim line from external sources. Thus, instead of reducing the effect of crosstalk or avoiding crosstalk effects by configuring the communication line in an appropriate way, preceding can be used to compensate for the effects of crosstalk on a communication channel. Precoding techniques are based on crosstalk channel information that includes both amplitude and phase information. Such information can be obtained from measurements such as slicer error or SNR. A particular example of such measurements for precoding is the use of pilot sequences and error feedback. The use of pilot sequences in G.vdsl2 or g.vector is described in contributions to ITU study group 15 question 4/15 such as those from Upzide Labs titled “G.vdsl2 pilot sequence assisted vector channel estimation” (C-0177, Geneva, October 2006) and Ikanos Communications titled “G.vdsl crosstalk channel estimation with improved convergence” (NC-082, Napa Valley, April 2007). The term error feedback, as used in this document, refers to a means by which the receiver of a communication system such as a CPE communicates to the transmitter of the communication system such as a DSLAM one or more received signal errors, or values derived from received signal errors. Received signal errors can be defined as the difference between the received signal and an estimate of the transmitted signal.
These papers describe the use of a particular pilot sequence on each communication line in order to estimate the crosstalk channels between the communication lines. For those skilled in the art it is clear that according to these papers, it is possible to use orthogonal pilot sequences for each of the communication lines in a precoding group. Thus, on each communication line a pilot sequence is transmitted which may influence other communication lines in the precoding group.
A pilot sequence, in particular in DSL technology, is a series of pilot signals which are transmitted for instance downstream from the Digital Subscriber Line Access Multiplexer (DSLAM) to the Customer Premises Equipment (CPE) and can for instance be described as a sequence of constellation points in a 4-QAM constellation. On a communication line, all active sub-carriers are modulated by two bits, either both zero (00) or both one (11), from the sync-frame using 4-QAM. The constellation points are rotated using a quadrant scrambler and due to the operation of the scrambler, it only has to be taken into account when crosstalk coefficients are determined. The constellation points 00 or 11 are transmitted downstream during a sync symbol on the communication line. Such sync symbols are periodically transmitted, typically after every 256 data symbols. The constellation points 00 and 11 have a complex representation being “+1+j” for 00 and “−1−j” for 11. For simplicity throughout this text, the constellation point 00 will be represented as ‘1’ and the constellation point 11 will be represented as ‘−1’.
A pilot sequence is a series of such constellation points, each transmitted in a separate sync symbol. For instance, a pilot sequence may consist of a sequence of 1 and −1 which is transmitted over the communication line, each sync symbol containing one of the points in the sequence. Since these points are defined and thus known to the receiver, it is possible to determine the difference between the expected symbol and the actual received symbol. This difference, which includes interference and noise can then be fed back to the DSLAM. The DSLAM is then able to correlate the error feedback and the pilot sequence in order to obtain the amplitude and phase of the crosstalk from one line to another line.
A first drawback of the existing solution is that the estimation requires a long period of time when the number of communication lines in the preceding group is high. Large precoding groups also require a large amount of processing power and storage to store and process all the error feedback information and to combine transmitted pilot sequences with the error feedback to obtain an estimation of the crosstalk.
A second drawback is found in a scenario where there are strong and weak crosstalkers in a precoding group. Some offender lines affect victim lines significantly more strongly than other offender lines. The influence of the weaker crosstalkers may therefore be irrelevant as their influence is negligible compared to that of the strong crosstalkers. The existing solutions devote equal time and computation to determining strong and weak crosstalkers and generally provide a complete estimation of all the crosstalk coefficients, which may be a waste of computational power and time because information from the strong crosstalkers alone may be sufficient to perform precoding in the preceding group.
A third drawback is that when a joining communication line is initialized, crosstalk estimates are required for that communication line and in particular the influence of that line on other lines in the precoding group is of importance. However the existing solutions are only able to estimate crosstalk for the entire group which means that when a line joins, it may take some time before sufficient information is available to configure the line and to perform accurate precoding.
When considering DSL lines, precoding is a new technique which is introduced to VDSL2 in the G.Vector extension thereof. However not all equipment supports the G.Vector specifications and thus not all communication lines are vector-enabled lines. Communication lines which only support the VDSL2 standard specification are called legacy lines. The sync symbols are defined in the VDSL2 standard specification for legacy lines. This means that transmitting a pilot sequence on a legacy line may conflict with the standard specification. Furthermore, legacy lines are generally unable to communicate errors measured on the sync symbols to the DSLAM. As a result, the existing systems have difficulties in obtaining crosstalk information in a precoding group which consists of both legacy lines and vector-enabled lines.
It is an objective of the present invention to provide a device for estimating crosstalk in a preceding group which is able to estimate crosstalk with reduced computational power. It is another objective of the present invention to improve crosstalk estimation in a precoding group with legacy lines and vector-enabled lines. It is yet another objective of the present invention to obtain information on lines of interest. It is another objective of the present invention to reduce the per-estimate delay.